The generator stator core is the largest monobloc component in a turbine-generator set. The stator core comprises thousands of thin steel laminations stacked horizontally and clamped together to form a cylindrical stator core disposed within a generator frame. Each lamination defines a central opening and thus when stacked an axial opening extends through the core. The laminations are held together by a plurality of axial through-bolts that extend from end-to-end through the core.
A rotor is disposed within the central opening and rotated by a rotating turbine. Electrical current is supplied to rotor windings such that rotation generates electric current in stator windings. The stator current is supplied to a plurality of main and neutral electrical leads mounted to the generator frame then to electrical loads through a transmission and distribution system.
Steady-state and transient forces generated during normal operation and transient conditions impose substantial forces on the stator core. These forces can distort the core geometric shape, cause the laminations to vibrate, and damage the core, rotor and/or frame. Also, mechanical fatigue caused by these forces can lead to premature failure of generator components.
Certain of these forces (including especially steady state forces generated during normal generator operation) may excite resonant responses in the generator and in particular in the coupling components that attach the core to the frame. Once the resonant response begins, the magnitude of these forces may increase substantially.
To reduce the effects of the steady sate and transient forces, the generator frame is fixed to a stable support such as the floor of a power plant and the stator core is solidly affixed to the generator frame. According to the prior art different attachment techniques and corresponding attachment components are employed to affix the core to the frame.
Keybars are used in one attachment technique. These long, axially-disposed members are located along an outer circumference of the stator core, specifically within slots defined in the outer circumference. The radially inwardly facing surface of each keybar is held within the slot by a geometrically capturing interfacing shape (for example a dovetail shape). A radially outwardly facing surface of each keybar is attached to the stator frame using various intermediate hardware components.
One such intermediate attachment component comprises a resilient spring bar. Several spring bars are distributed circumferentially around an interior surface of the frame and each spring bar extends axially through the frame. A first surface of each spring bar is attached to radially inwardly facing generator frame ribs and an opposing second surface of each spring bar is attached to a key bar mounting block or plate. The key bar block or plate is attached to the keybar.
The end of each keybar (both the exciter end and the turbine end) comprises a threaded segment for receiving a threaded nut and mating washer. The nuts are tightened to provide a clamping force to the stator core.
FIG. 1 is a partial cutaway perspective view of a prior art electric generator 8 and a stator core 10 mounted within a generator frame that is not shown in FIG. 1. FIG. 1 further illustrates a spring bar 15; a plurality of such spring bars 15 is distributed around a circumference of the core 10. Each frame ring 13 comprises a circumferential component 13A and a transverse component 13B.
A first surface of each spring bar 15 is attached to a plurality of the transverse components 13B by fasteners 19. Each transverse component 13B is welded to a circumferential component 13A and each circumferential component 13A is welded to an inside surface of the generator frame.
Each spring bar 15 extends an axial length of the core 10. At a plurality of axially spaced-apart locations a second surface of each spring bar 15 is attached to a key bracket or key block 20 using fasteners 18. Each key block 20 spans a width of a keybar 22 and a plurality of key blocks 20 are axially distributed along each keybar 22.
The keybars 22 are fixedly captured within the core 10 by a geometrically capturing interface defined in an outer surface of the core 10. The keybars 22 and the core grooves are shaped such that the keybars 22 are captured within the groove by the geometric capture feature, such as the illustrated dovetail shape. A fastener 7 is tightened to provide additional forces to secure the keybar 22 to the core 10. Thus the core 10 is connected to the generator frame by serial coupling of the keybars 22 geometrically retained within core grooves, the key blocks 20 and the spring bars 15.
Stator windings (also referred to as stator bars, but not illustrated) are disposed within winding slots 21. Through-bolts extend axially through openings 23. The through-bolts and mating nuts (neither illustrated in FIG. 1) cooperate to exert inwardly-directed axial clamping forces on core end plates and laminations that comprise the core 10.